1. Technical Field
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that employs a belt for fixing images, and an image forming apparatus incorporating such a fixing device.
2. Background Art
In electrophotographic image formation, an image is formed by attracting toner particles to an electrostatic latent image on a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium to obtain a print output.
One specific type of the fixing device is a roller-based fixing device employing a pair of cylindrical fixing rollers, one being a fuser roller subjected to heating, and the other being a pressure roller disposed opposite the fuser roller. The pressure roller presses against the fuser roller to form a fixing nip therebetween, through which the recording sheet is conveyed. At the fixing nip, the fuser roller heats the incoming sheet to fuse and melt the toner particles, while the pressure roller presses the sheet against the fuser roller to cause the molten toner to set onto the sheet surface.
Another, more thermally efficient fixing device employs a flexible, looped fuser belt, instead of the fuser roller, subjected to heating and disposed opposite the pressure roller. Compared to the roller-based configuration, the belt-based fixing device does not require significant time to heat the fuser assembly to an operational temperature upon start-up, owing to a relatively low heat capacity of the fuser belt lower than that of the fuser roller.
FIG. 1 is an axial, cross-sectional view of an exemplary belt-based fixing device 120.
As shown in FIG. 1, the fixing device 120 includes a fuser belt 121 looped into a generally cylindrical configuration, a fuser pad 126 disposed inside the loop of the belt 121, and a pressure roller 131 pressing against the fuser pad 126 via the belt 121 to form a fixing nip N therebetween. Also included is a stationary, tubular pipe 122, formed of metal or heat conductive material, around which the fuser belt 121 is supported or guided during rotation. A heater 125 is disposed inside the tubular pipe 122 to radiate heat to the pipe 122, which in turn conducts heat to the fuser belt 121 entrained around the pipe 122.
One problem associated with the belt-based fixing device is that the recording medium, in particular, a sheet of paper that contains relatively large amount of moisture, curls or bends toward the fuser belt in a manner similar to that of a bimetallic strip. Such curling causes the recording sheet to eventually wind or wrap around the fuser belt upon exiting the fixing nip, leading to malfunction or even failure of the fixing process.
To counteract the problem, a technique has been proposed that employs an anti-curling member downstream from the fixing nip to prevent curling and concomitant winding of the recording sheet around the fuser belt.
According to this method, the fuser belt comprises a looped film supported around a guide member within which a heater is accommodated. The anti-curling member is a protrusion formed integral with the guide member, which contacts the recording sheet upon exiting the fixing nip, so that the outgoing sheet moves away from the fuser belt. Disposed downstream from the anti-curling member is a sheet stripper, which defines a longitudinal edge adjacent to and out of contact with the fuser belt to engage the leading edge of the recording sheet for separating it away from the fuser belt.